The present invention relates generally to continuous papermaking operations and is more particularly concerned with a new and improved papermaking process wherein the fiber dispersing media is fully recirculated within the system.
As is well known, in a wet papermaking process the operating conditions and characteristics required for forming an aqueous fiber dispersion differ substantially from the optimum conditions needed for forming the fibrous web material. For example, a high fiber consistency is preferred at the chest or fiber dispersing area of the system while it is generally desired to provide a very dilute fiber dispersion at the headbox or web-forming area of a papermaking machine. Similarly, a low viscosity dispersing media at the web-forming area will usually promote rapid drainage while a high viscosity dispersing media frequently is needed to prevent entanglement and achieve good dispersion of the fiber stock prior to its introduction into the headbox of a papermaking machine. Accordingly, it is generally preferred to provide conditions of high viscosity and high fiber consistency when initially dispersing the fibers and low viscosity coupled with low fiber consistency at the headbox and sheet-forming area of the papermaking machine.
As set forth in U.S. Pat. No. 3,093,534, the fiber furnish or dispersion is frequently formed at maximum viscosity, using an additive such as polyacrylic acid at a fiber consistency of up to about 6 percent by weight. The furnish is then diluted with water to a fiber consistency of about 0.1 to 1.0 percent fiber at the headbox prior to its application to the web-forming screen. The diluating operation that lowers the fiber consistency also substantially reduces the concentration of the viscosity producing agent such that the aqueous solution of the agent cannot be reused for fiber dispersing purposes without replenishing substantial quantities of the agent. Thus much of the economic advantage associated with a recirculating system is lost.
The desirability of providing a continuous recirculating system for the "white water" of a papermaking system has long been recognized and one such attempt is described in U.S. Pat. No. 3,245,871. That patent indicates that prior agents were substantive to the fibers and suggests the use of nonsubstantive water soluble hydroxyethyl cellulose and high molecular weight derivatives of glucose in place of the prior viscosity producing agents. However, even in such a system it is required that the fiber dispersion be diluted at the headbox with fresh water to achieve the requisite fiber consistency and additional quantities of viscosity producing agent must be added to compensate for the dilution.
Accordingly, it is an object of the present invention to provide a new and improved continuous recirculating papermaking system well suited for high viscosity, high fiber consistency within the fiber-dispersing area of the system and low viscosity, low fiber consistency at the web-forming area without the necessity for altering the concentration of the viscosity producing agent during the entire recirculatory process.
Another object of the present invention is to provide a recirculating papermaking process of the type described that utilizes viscosity producing agents which are responsive to readily adjustable chemical and physical changes in order to alter the viscosity of the fiber dispersing media. Included in this object is the provision for use of agents capable of altering the viscosity of the media in response to changes in the pH of the media.
Yet another object of the present invention is to provide a substantially closed recirculating papermaking system or process that exhibits the economic advantages associated with the conservation of a viscosity producing agent through the repetitive recycling thereof. Included in this object is the provision for viscosity producing agents that are sensitive to changes in pH in a repetitive and recirculatory fashion in order to adjust and control the viscosity of the material within which they are dispersed and provide optimum operating conditions at different locations within the system.
A further object of the present invention is to provide a process of the type described that utilizes aqueous solutions of polymers and gums capable of altering the viscosity of aqueous dispersing media in response to reversible changes in the pH of those solutions.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are accomplished in accordance with the present invention by providing a continuous, essentially closed recirculating papermaking process that includes the steps of forming a dispersion of fibers within a medium containing a viscosity producing agent, reducing the fiber consistency and the viscosity within the dispersion while maintaining the concentration of the agent substantially unchanged, subsequently forming a fibrous web material on a papermaking screen from said dispersion of reduced fiber consistency while separating the dispersing medium from the fibers forming the web and continuously collecting the separated medium and recirculating it within the system to effect both the formation of a subsequent fiber dispersion and a reduction in the fiber consistency and viscosity of a subsequent fiber dispersion without substantially changing the concentration of the viscosity producing agent, said separated and recirculated dispersing medium being adapted at the time of forming the subsequent fiber dispersion of forming a high viscosity dispersing medium for the fibers.
A better understanding of the invention will be obtained from the following detailed description of the several steps and the relationship of one or more steps with respect to each of the others as well as the features and properties exemplified in the following detailed description.